Latex-based paints have captured a significant portion of the indoor and outdoor paint market as a result of the many advantages that such paints have over solvent-based products. The main advantage of latex-based paints include easy clean up, low odor and fast dry.
Two main types of emulsion polymers are employed in formulating latex paints, namely (1) acrylic emulsion copolymers, comprising of alkyl esters of acrylic and methacrylic acid, or combinations of such esters with vinyl aromatic co-monomers like styrene, along with minor amounts of acrylic and methacrylic acid, and (2) vinyl acetate copolymers, comprising vinyl acetate in combination with ethylene, vinyl chloride and/or minor amounts of softer acrylic monomers. The acrylic emulsion polymers are mainly used in premium quality paints for excellent water resistance, desired levelling, film hardness and scrub resistance. The vinyl acetate polymers exhibit toughness and scrubability but poor water resistance. Vinyl acetate polymers tend to be much less expensive than the acrylic polymers
Notwithstanding the advantages of VAE-based latex paints, it is well documented that several deficiencies exist for VAE emulsions in paint products versus solvent-based paints and acrylic-based latex paints. One such deficiency relates to wet adhesion. The term “wet adhesion” is used in the paint industry to describe the ability of a paint coating to retain its adhesive bond under wet conditions. Good wet adhesion is well known in solvent-based paints, but water-based paints tend to lose adhesion in wet or humid conditions. This inherent deficiency limits extensive use of latex paints in humid environments. For example, paints intended for exteriors are frequently exposed to moisture, due to rain and humidity, while the same harsh conditions are encountered in interior paints used in bathrooms and kitchens. Further, resistance to washing and abrasive scrubbing also is an important requirement for painted surfaces which become soiled and must be washed and cleaned.
Much effort has been devoted in the past to improving the wet adhesion capabilities of latex-based paints. This effort has involved attempts to optimize various paint formulation parameters, such as pigment types, dispersant types, surfactants and coalescing agents. The most significant improvement in wet adhesion properties has been observed through functional modification of the polymer backbone of the latex binder, to incorporate amine, amide and acetoacetate functionalities.
In particular, cyclic ureido derivatives have been described in the patent literature as imparting wet adhesion properties, and such components are described, for example, in U.S. Pat. Nos. 4,104,220; 4,111,877; 4,219,454; 4,319,032; and 4,599,417. However, cyclic ureido-containing functional monomers are very expensive and their inclusion results in a drastic and uneconomic increase in the cost of the vinyl acetate polymers. U.S. Pat. No. 3,935,151 describes another approach to improving the wet adhesion properties of vinyl acetate polymers by blending into a vinyl chloride/acrylic latex a copolymer which is a vinyl-acrylic, a vinyl chloride-acrylic or an all acrylic latex containing hydroxy methyl diacetone acrylamide (HMDAA).
Another deficiency of VAE-based latex paints in general versus solvent-based paints and acrylic-based latex paints relates to block resistance. Dry paint often comes into contact with itself especially in window and door areas and, depending on its hardness, the pressure, temperature, humidity, and duration of time which the surfaces are in contact, the painted surfaces sometimes stick together. This undesirable sticking together of two painted surfaces when pressed together or placed in contact with each other is referred to as “blocking”. Thus, an important characteristic of coatings is block (or blocking) resistance.
One approach for improving block resistance of paint films based on VAE latex technology has been to blend into the VAE latex-based compositions a certain type of acrylic latex particles containing trialkoxylated silane functionalities as set forth in U.S. Pat. No. 6,174,960. This is deemed to be a relatively expensive route to enhanced blocking resistance performance due to the cost of the trialkoxylated silane monomer.
Notwithstanding the above-described prior art techniques for improving wet adhesion and block resistance of VAE emulsion-containing, aqueous latex paints, there continues to be a need to identify new coating formulations, especially in the form of paints, which employ VAE film-forming lattices and which also provide enhanced paint film wet adhesion and anti-blocking performance. It has been found that an improved VAE based coating composition, e.g., a latex paint, can be produced by blending into a VAE-based paint formulation an acrylic latex copolymer emulsion wherein the acrylate copolymer contains selected types and amounts of modifying co-monomers and is used with a selected type and amount of a specific cross-linking system.